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lighting

Hello everyone.
I have imported an obj file(with materials in .mtl) into the OpengGL, it works well when I use the glperspective with lighting ,looking like 3-dimentional ...but when I use the glOrtho ,it just is totally black with lighting. I figured sth. wrong with my light postion,so I changed the light position and nothing happened. And when I don't use lighting, it looks like sth. planar.
Can some guys help me?
And any help will be appreciated, besides, forgive my poor English~~